The present invention relates to abradable shroud assemblies for use in turbomachinery, such as gas turbine engines. More particularly, this invention relates to a geometric profile of a containment case in regions forward of where a rotating blade row is aligned.
In most turbofan engines the fan is contained by a fan case that is equipped with a shroud. The shroud circumscribes the fan and is adjacent to the tips of the fan blades. The shroud serves to channel incoming air through the fan so as to ensure that most of the air entering the engine will be compressed by the fan. A small portion of the air is able to bypass the fan blades through a radial gap present between the tips of the fan blades and the shroud. Conventionally, the radial gap is very narrow such that the amount of air that is able to bypass the fan through the gap is limited. The efficiency of the engine can be significantly improved in this way.
Because the gap is narrow, the fan blades may rub the shroud during the normal operation of an aircraft turbofan engine. An abradable material is configured into the shroud for this purpose. However, any rubbing contact between the tips of the fan blades and the shroud will tend to cause the fan blades to deflect and eventually become unstable. Such rubbing events can be self-feeding as the blade continues to deflect during the event. Therefore there is a need for a shroud that is configured to minimize the frequency and severity of rub events during operation.